Hydraulic pressure systems, as used in motor vehicles, require a supply of transmission fluid. Maintaining an adequate oil level in the transmission oil pan at all operating temperatures is essential for effective transmission performance. The oil pan serves as a sump to provide the necessary supply, however, in front wheel drive vehicles the transmission oil pan is too shallow to hold an appropriate quantity. The problem is exacerbated by the fact that the volume of the oil used expands at elevated temperatures. The height of the level of the sump needs to be limited in order to avoid having moving elements of the transmission from being immersed causing the creation of foam which would result in loss of torque. On the other hand, if the sump level falls too low, there would be a loss of pump inlet supply which would result in damage. The problem has been dealt with by employing an auxiliary sump at the side of the transmission case cover. The fluid level in the auxiliary sump is controlled by a thermostatic element which controls the amount of force placed on a valve element controlling the amount of fluid flow from the auxiliary sump to the main sump in dependence upon the temperature of the oil in the auxiliary sump. As the temperature of the oil increases, the force placed on the valve element by the thermostatic element increases to produce an increase in the height of the oil level in the auxiliary sump in a controlled manner ideally maintaining the height of the oil in the main sump at a constant level.
A control valve of the type described is shown in U.S. Pat. No. 5,195,678, assigned to the assignee of the present invention, the subject matter of which is incorporated herein by this reference. A control made in accordance with an embodiment of that patent is shown in FIG. 10 and comprises a generally U-shaped member 120 comprising a strip 126 of thermostatic material having a mounting strip 122 of monometal at a first end mounted on a first member 104 of a pair 104, 106 of thermoplastic post members extending upwardly from a base 102. The integrally attached thermostatic strip has a second opposite end 134 biased against a metal cover plate 110 covering a fluid aperture 108, the cover plate being slidably movable along the post members toward and away from the fluid aperture. A first fluid path is provided at temperatures below a selected level in which the cover plate hinges about a location on the second member 106 of the pair of post members and a second fluid path is provided at temperatures above the selected level with the effective hinge location shifting to a location on the base adjacent the first post member 104. A spring 136 is disposed on the second of the pair of post members which places a bias on the cover plate so that a force relatively independent of temperature is placed on the cover plate at one end while another force dependent on temperature is placed on the cover plate at a location on the other side of center line between opposite ends of the plate with the result that the effective location of the hinge point switches between two different locations to provide a non-linear curve of head height (i.e., force) versus temperature.
The control valve needs to respond to different flow rates, for example, from a low of 1/4 gpm at idle to 1 gpm at full throttle. With reference to FIG. 10, a target, or model curve of fluid height in mm in the auxiliary sump versus temperature for a particular automotive transmission is shown in FIG. 8 with curve a representing the 1/4 gpm model and curve b representing the 1 gpm model. The model curves, showing an increase height of the auxiliary sump with increasing temperature, result in an essentially constant oil level in the main sump in the selected transmission. A control valve made in accordance with the FIG. 10 embodiment of the above referenced patent resulted in actual height shown by curve c for 1/4 gpm and d for 1 gpm. Although the control valve resulted in tracking of the model curves, it would be desirable to decrease the difference or delta between the model and actual curves, particularly in the 1/4 gpm low and middle temperature operating range. It would also be desirable to decrease the differences between the low flow rate and high flow rate curves.
Additionally, it would be desirable to provide a control valve which is less expensive to manufacture and assemble and one which has improved longevity. For example, in the FIG. 10 embodiment, each device must be separately calibrated by pushing the temperature responsive member onto its respective post to a selected location and the coil spring member onto its respective post to a selected location as determined by a defined force level. Although further adjustments can be made in pushing the members further down their respective posts, care must be taken not to exceed the calibrated positions to avoid having an unusable device. Another limitation is that over time the plastic posts tend to yield or bend causing the calibration to drift. Still another limitation is that the temperature responsive member is relatively complex using a monometal portion welded to a thermostat metal strip formed into a somewhat precise U-shape.
From an ideal standpoint, the target curves could most easily be achieved by using an elongated, narrow thermostat blade having a low mechanical spring rate; however, in certain transmission housings there is insufficient space for this approach. With transmission housings to which the present invention is addressed, the available footprint is more square shaped than elongated and on the order of only one to two inches in length and width.